WHY WE WENT.
The Professor's story. — The noblest problem in Astronomy. — Kepler's law of the Distances. — Sun's Distance the standard of celestial measure. — How celestial distances are measured. — Mathematics and knitting-needles. — The Transit of Venus. — The Opposition of Mars. — Lieut. Gilles's disappointment. — The Astronomer Royal's plan. — My husband's proposal. — Ascension fixed upon. — A grant from the Astronomical Society. — Lord Lindsay's Heliometer. — The Heliometer at Burlington House. — Accident to the Heliometer. — Ready to start.
I REMEMBER a story once told me by a learned friend. He had been explaining to a lady, with much care and minuteness, the reasons why the axis of the earth is slowly though constantly changing its direction in the heavens, and why, therefore, the star, which is the Pole star now, was not the Pole star 4000 years ago.
The lady had encouraged our friend to proceed with his explanation by the most marked attention, and by such appreciative interjections as "Really!" "Indeed!" "How beautiful!" In this way he was led to more than usually minute description, and with much unction proceeded to crown his argument as follows.
"Now you see, by this change of the direction of the earth's axis, if we have any permanent record of an observation of the angular distance of a star from the Pole, we can calculate how long ago that record was made." "Of course!" "And in the Great Pyramid we have such a record." "Indeed! how wonderful!" "The entrance passage points to the north, and its angle of inclination corresponds with the lower culmination of the Pole star of 4000 years ago."
Here a little hand was laid on our friend's arm, and his feelings may be better imagined than described, when, in an anxious voice, the question was put, "And pray, Professor, what is an angle?"
Now, I too have a story to tell in which angles occur, and, warned by the Professor's experience, I would leave them out if I could. This, however, I cannot do altogether, lest some should thus miss the point of the story ; but, as next best, I shall throw them all into the first chapter ; and those of my sisters who care for none of these things, or who, like the Professor's fair friend, know not the meaning of an angle, may pass it over and read about our "Six Months in Ascension," without the reasons that took us there.
It was no longing for new scenes, no thirst after gold, no need for better health that led us to bid good-bye to England in the leafy month of June, and seek a barren rock at the Equator. It was none of these things. We went in search of the Solar Parallax, or in other words to find out how far off the sun is from the earth.
Many a noble head has puzzled over this problem, and many a sage thought and many an hour of careful observation did the grand old philosophers give to its solution. Our own Astronomer Royal, in a paper read in 1857, says: "The measure of the sun's distance has always been considered the noblest problem in astronomy." The general interest taken in the last Transit of Venus, and the large sums expended by different nations in providing for its observation (amounting in the aggregate to about a quarter of a million sterling) show that the solution of the question still maintains its importance, not only as the settlement of an abstract truth, but as an essential condition to the future progress of astronomy ; and I think I can show why this is so.
The astronomer knows, and has known for ages, the proportional distances of all the bodies of our solar system ; he knows too, as a mathematical fact, that there is an exact relation between the time which a planet takes to make a complete revolution round the sun and the distance of that planet from the sun. Now it is easy to find the time which a planet takes to go round the sun, and the knowledge of the old astronomers in this respect was nearly as accurate as that of our own day.
When Kepler discovered his famous law of the distances, he was in a position to draw nearly as accurate a chart of the sun and the paths of the heavenly bodies as we could draw at the present time. What he could not do was to give the scale of his chart. He could say at any time, "If you can tell me what is the distance between any two planets I can tell you the distances of all the others, because I know exactly the relative proportions of all these distances."
He was, in fact, in the position of a man who has a map of England placed before him and is requested to tell from it the distance from London to York. If the map has no scale attached to it he cannot do so : but if he is told that one inch on the, map is equivalent to ten miles, he has only to measure the number of inches between London and York on the map and to multiply the result by ten, to find the distance in miles.
Or again, if he happens to know any other distance on the map, such as the distance from London to Oxford, he has only to find how many times this distance is contained in the distance from London to York by the map, and thus he solves the question.
Accordingly, the astronomer requires only to know one distance in our solar system in order to know all ; and for this purpose he selects as his unit of measure the mean or average distance of the earth from the sun. Thus we do not order 9000 millimetres of silk for a dress, nor astonish the draper by demanding 1/176 of a mile ; but prefer to ask for 10 yards of silk. So the astronomer, instead of saying that the average distance of Jupiter from the sun is 478 millions of miles, prefers to say its distance is 5 1/5 meaning that it is 5 1/5 times the distance of the earth from the sun.
The determination, then, of the length of the astronomer's unit, is of the same importance to him, as is the true length of the yard measure in the ordinary business of life, or in the more scientific work of the surveyor or engineer. In order to accomplish this determination, he has, as I have shown, only to find the distance of any one planet from another ; and now I must explain how this is done.
Many will remember the great meteor-shower of 1866. On that occasion, one very remarkable meteor was seen by an observer in Aberdeen to burst in the South, apparently near a well-known star in the constellation of the Bull, while to another observer in Newcastle the same meteor appeared to burst in the North, near another well-known star in the Great Bear. The time of bursting, and the angular distances of the stars (from the Pole and from the meridian) being known, it was easy for the astronomer to calculate the apparent altitude and direction of these stars as seen from Aberdeen and Newcastle respectively.
But without the employment of mathematical terms, it is difficult to explain how he works ; and with them, it would be hopeless for me to attempt the explanation. I myself do not understand mathematical terms, so how could I use them with the hope of explaining these things to my readers? However, I can use knitting-needles, and perhaps they may do just as well.
Let us suppose that the astronomer takes a map of England and places one end of a knitting-needle on the town of Aberdeen, then turning the other end of the needle in the proper direction he raises it to the required altitude for the star in the Bull. Similarly, he takes another needle, places one end on Newcastle, turns the other end in the direction of the star in the Great Bear, raises it to the required altitude, and where the needles cross each other, there must be the place of the meteor. In the case in question it was found to be 40 miles vertically over the town of Dundee.
Of course the astronomer uses no such clumsy contrivances as knitting-needles. He finds the lines of the mathematician more convenient, but the principle is the same. To measure the distance of any celestial body from the earth, it is only necessary to observe it from two different points. In the case however of measuring the great distance of a planet, the problem becomes very difficult. For even when the planet is looked at from opposite sides of the earth, the lines (or needles) must go so far before they meet, that the angle at the apex is almost insensibly small, and yet on the measurement of this minute angle (called the parallax) does the whole problem depend.
The Transit of Venus had been supposed to afford the most accurate means of measuring a planet's distance ; because at a Transit of Venus the planet is only at about one-fourth of the sun's distance from the earth, and passes across the sun between it and the earth. At that time, when the edge of the planet seems to an observer on one side of the earth to touch the edge of the sun, to an observer on the opposite side of the earth this does not appear to be the case, because of the apparent change in the planet's position, produced by its being viewed from different points.
In this way, to observers situated at different points of the earth, the edge of Venus appears to touch the edge of the sun at different times, and from the difference of these times astronomers can calculate the angular change in the planet's apparent position. Then, since they know the size of the earth and the latitudes and longitudes of the observers' stations (and consequently their distance apart), they are able to calculate the distance of Venus in the same way that the distance of the meteor was calculated.
But a somewhat unexpected difficulty was found in the observations of the Transit of Venus, It was discovered that the planet Venus was surrounded by a dense atmosphere, so that the sun's edge seen through it was hazy and indistinct. The observations consequently were not made with the precision that was necessary ; and it became desirable to find some other method of settling the great problem.
Now, it happened that during August and September of 1877, the most favourable Opposition of Mars possible in the present century would occur. An "Opposition of Mars" occurs when that planet, the earth and the sun, are nearly in a straight line, the earth being between the planet and the sun. Hence the planet comes to the meridian at midnight. In the case of Mars, this condition of things is realized nearly every two years, but at the Opposition of 1877 he would be nearer to the earth than at any Opposition during the present century, and on the 5th September he would be only one-third of the sun's distance from the earth.
Under these circumstances my husband proposed a method of observing the planet which he believed to possess advantages over all other methods. Instead of employing two sets of observers in different parts of the earth, as in the Transit of Venus, he determined to combine both sets of observers in himself. He would thus avoid the disappointment that occurred to Lieut. Gilles, who went to Chili in 1850 and made laborious observations for the parallax of Mars, but found on his return that hardly any corresponding observations of importance had been made, in the northern hemisphere. Thus his labour had been expended in vain. The way in which my husband managed to avoid the possibility of a like catastrophe was as follows.
He proposed to observe the planet in the evening when it was rising—in other words, to look at it from position A ; then to observe it in the early morning when it was setting, that is to say, to observe it from position B. He availed himself in fact of the rotation of the earth to carry both himself and his Observatory round, and so, by merely waiting, to be transported 6,000 or 7,000 miles between the times of his evening and morning observations.
This part of the proposal was not original. It had been suggested long ago by the Astronomer Royal, but had never been acted on. My husband, however, proposed for the first time the method of observation which he carried out, viz., the employment of the Heliometer—the most exact of all angle-measuring instruments—and, to secure accuracy in the result, he elaborated details which need scarcely be described here.
When his scheme was ripe, he drew it up in complete form, and it received from the Royal Astronomical Society, from the Astronomer Royal and others, the most cordial support. A sum of 500l. was granted from the funds of the Royal Astronomical Society in April, 1877, in order that be might carry out an expedition to the Island of Ascension, to observe the Opposition of Mars in the following autumn.
This expedition, however, no money could have rendered possible at this date, had not Lord Lindsay, with the greatest kindness, agreed to lend for the purpose his splendid Heliometer. This is the only instrument of the kind in England except the much larger one in the Radcliffe Observatory, Oxford, and it was not available.
My husband decided on Ascension as the most suitable station for making the desired observations, on account of its favourable position with regard to latitude, and its reputed meteorological conditions. Through the good offices of the Astronomer Royal, the consent of the Admiralty was obtained for our occupying this station.
The Transit Instrument was lent by the Royal Astronomical Society. Some considerable modifications were necessary ; but these were duly made. The Astronomer Royal lent the Transit Hut ; the Heliometer Observatory, a chronograph, five chronometers, two reflecting circles, some barometers and thermometers, completed our instrumental equipment. The whole, in their packing cases, together with our personal luggage, made up about 20 tons measurement of baggage.
But before starting, very particular attention was required in regard to the Heliometer—the keystone on which the whole structure of the work rested. And here begins the story of its adventures and mischances.
The instrument had never been used in so low a latitude as Ascension, and it was necessary to test it carefully, in order to ascertain whether it would perform its functions well under the untried circumstances.
Considerable interest in the expedition having been shown by members of the Royal Astronomical Society, it was thought best to erect the instrument in their rooms at Burlington House, where the necessary trials could be made, and that the instrument might afterwards be exhibited and explained d at one of the evening meetings. The Heliometer was duly erected, and all had been brought nearly into the same condition of affairs as would be required at Ascension. David was applying a level to an inclined piece of wood cut to the angle of the latitude of Ascension, and was directing the workmen to give a final motion. to the screw by which the inclination of the axis is changed, when slip ! the screw gave out, the overhanging weight of the Heliometer and its counterpoises tore the lower end of the cradle from his hand, and, tilting upwards, the polar axis, counterpoise weights and Heliometertube, in all several cwt., came down crash, from a height of 7 or 8 feet, upon the floor.
Imagine the astronomer's feelings as he saw the Heliometer of all his hopes light upon its delicate eye-end ; that eye-end driven through the floor and slowly torn off, as the whole mass gradually turned round, smashing and crushing the more delicate rods, handles and other attachments to the tube, and finally squashing one of the copper caps which protect the ends of the slides from dust.
As the whole thing lay there on the floor, within ten days of the time when it must be packed for shipment, it seemed impossible that it could be restored fit for use. The apparent ruin of so many hopes and plans was paralysing, and for some minutes David was quite incapable of examining the amount of damage done. By-and-by, however, as he came to look into details, matters did not prove to be so desperate as they had at first sight appeared. The tearing and smashing and crushing of the eye-end, handles, &c., had had the happy effect of breaking the fall ; and on removing the head, he was delighted to find that the object-glass, the slide, the scales, and in fact all the really vital parts of the Heliometer proper were intact, and working as smoothly and beautifully as ever.
The life was still there, and the shattered limbs were at once placed under the care of able surgeons, who in six days made them whole as before. Messrs. T. Cooke & Sons, the great opticians of York, Mr. Browning, and Messrs. Troughton and Simms of London, were all pressed into the work, and with a will they accomplished it. But what a time of strain it was, and how tired we were before we started! Yet all the while we never ceased to congratulate ourselves on the misfortune having taken place when and where it did.
The cause of it was simply that the elevating screw was too short, and the instrument being called a "Universal Equatorial," that is, adapted to all latitudes, this deficiency could not have been anticipated. Had it not been for this trial in Burlington House, in all probability, a like accident would have happened at Ascension, the result of which would simply have meant the utter failure of the expedition.
It was only at the last moment that we were ready ; but we were ready. The evil that is past is not to come.
The squares of the times of the revolutions of the planets are proportional to the cubes of their mean distances from the sun.